The present invention relates to a method for exchanging rechargeable batteries to rebuild an assembled battery, which includes a plurality of rechargeable batteries that accumulate memory effect, by exchanging rechargeable batteries that have become defective with replacement rechargeable batteries.
In electric vehicles, hybrid vehicles, and the like, an assembled battery including a plurality of rechargeable batteries is used as a power source. In such an assembled battery, due to differences between rechargeable batteries or defective parts used in a rechargeable battery, some of the rechargeable batteries in an assembled battery may have a shorter life than the other rechargeable batteries. When such a problem occurs, the capacity of the assembled battery cannot be used to full extent. This may cause an anomaly in a system that uses the assembled battery. If the entire assembled battery were to be exchanged with a new one, the rechargeable batteries that are not defective would be wasted. Accordingly, studies have been conducted to rebuild an assembled battery by exchanging only defective rechargeable batteries with normally functioning rechargeable batteries and reusing the other rechargeable batteries that do not have to be replaced.
A rechargeable battery typically used in an electric vehicle or a hybrid vehicle may be a nickel cadmium (NiCd) or nickel metal hydride (NiMH) rechargeable battery. Such types of rechargeable batteries are susceptible to the so-called memory effect. Memory effect causes a difference in the characteristics of the rechargeable battery between initial use and after use over a certain period. As the use of the rechargeable battery continues, the accumulated memory effect increases. The accumulation of the memory effect is dependent on the usage environment and usage conditions and thus differs between users. Thus, conformance with such used rechargeable batteries (i.e., non-exchanged rechargeable batteries) must be considered when rebuilding an assembled battery. Otherwise, the rebuilt assembled battery may have insufficient capacity.
Japanese Laid-Open Patent Publication No. 2004-185915 and Japanese Patent No. 3820184 describe examples of solutions to such technical problems.
The '915 publication suggests a rechargeable battery exchanging method that charges replacement rechargeable batteries to a state of charge (SOC) of which level is lower than the SOC of non-exchanged rechargeable batteries and then exchanges the replacement rechargeable batteries with originally installed rechargeable batteries. In an assembled battery that is rebuilt in this manner, by repeating charging and discharging, the difference in SOC between the newly installed replacement rechargeable batteries and the non-exchanged rechargeable batteries becomes small. Consequently, the SOC becomes the same in all of the rechargeable batteries forming the assembled battery. Thus, as described in the '915 publication, the capacity of the assembled battery may be used to full extent.
The '184 patent suggests a rechargeable battery exchanging method that uses replacement rechargeable batteries subjected to memory effect at a level that is about the same as the memory effect accumulated in the non-exchanged rechargeable batteries. In an assembled battery that is rebuilt in this manner, the voltage characteristics of the replacement rechargeable batteries become the same as the non-exchanged rechargeable batteries. Thus, as described in the '184 patent, the capacity of the assembled battery may be used to full extent.